Panel meter wiring

[thomasdj]

Hi!:)

I just bought a LCD panel meter to install in my amp to measure cathode voltage..but how to hook it up?? I know this is properly an odd question but I have no experience in these things..

It has 13 pins:

1 = V+
2 = V-
3 = NC
4 = NC
5 = COM
6 = INLO
7 = INHI
8 = RFL
9 = RFH
10 = ROH
11 = D1
12 = D2
13 = D3

It’s a common ground, 200mv meter.

What to connect to what?

Thanks

Thomas

[Enzo]

Where did you buy the meter? Did it come with a spec sheet? If not can you find a spec sheet from the manufacturer? I assume it needs a power supply to operate.

[thomasdj]

Hi Enzo,

Here is a link to the spec..

http://www.circuitspecialists.com/images/CX101A&B&BG.pdf

CX101B, common ground

It is not a problem to power the meter..I use a 7805 regulator.

But I don know which pin should be connected to measure the milivolts. I won't to build the meter into the amp (share ground with the 1 ohm cathode resistor) and then measure the milivoltage, just like the geofex "diy bias probe" (except from that I use pin 8 of the power tubes instead of pin 3).

Thanks

Thomas

[Steve Conner]

I recommend connecting INLO and INHI to the ends of the cathode resistor. Right at the resistor, so you get an accurate reading. INHI to the cathode end and INLO to the ground end.

As for the power supply, read the spec sheet for the suggested hook-up. I seem to remember that you apply your 5V between V+ and COM, and by "common ground" they mean that COM is allowed to be connected to INLO. (In fact, some sort of connection between COM and one of the inputs is probably mandatory.) I believe the meter generates its own internal supply on V-.

I'll leave it as an exercise to the reader to figure out what pins 8-13 do :rolleyes:

[PRNDL]

I believe you may also need to set the decimal point, otherwise it shows 20 (mA or mV) as 200.

I've been considering what would be an optimal design for a bias meter, and have some questions. I'd like to use two meters to show bias for matching.

Are there any advantages to using an analog meter over digital?

Do analog meters also need a 1% 1 ohm external resistor (or are they built in)?

Is it better to mount the 1 ohm resistor in the box or in the socket?
(It would be easier to mount in it the box).

Is there a wire pair that holds up to constant movement for connecting the probe to the box?

Thanks!

[Enzo]

The 1 ohm resistor is for sensing, it is not part of the meter. VOltmeters and the circuits they measur would not be very happy with a 1 ohm resistor across them. Can you imagine measuring the B+ of a tube amp with a 1 ohm resistance across the meter?

The reason we use 1 ohm is so the math is simple. We are measuring current after all, so across a 1 ohm resistor we measure voltage, and since the resistance is 1, we find the milliamps correspond to millivolts across the resistor. You could just as easily use a 17 ohm resistor, but the arithmetic would become stupid.

Analog meters will give less accurate readings, but that lack of accuracy will be fractions of a millivolt. I am assuming reasonable size meters now, not 6" Simpsons. But analogs will give instant visual indications of trends. WIthout having to think what the numbers mean, you can instantly spot the needles pointing different directions.

Wire? Do you really use it that much? The cathode wire carries well under 1 volt, and rarely carries even a tenth of an amp, so almost ANY wire will do. Guitar cord or mic cord wire ought to last in the flexing area. Doesn't need shieding, so plain old power cord or zip cord would work. If you include a wire from the plate that changes things. The plate wire needs to have 600v insulation. But you could slap a piece of that next to some guitar cord and sleeve it with shrink tube.

[PRNDL]

The bias meter I have has several issues that I'd like to address with my new design.

It looks like they used telephone wire, which isn't very flexible.
The voltage reading is off more than the calibration pot can handle
It's a digital LED, and I'm thinking about going analog


A digital LED is very easy to read and gives what appears to be accurate readings (i.e. numbers), but I suspect that inexpensive ones aren't very accurate (primarily because this one is off).

The accuracy does depend on the accuracy of the resistors, which may also be off in this meter.

Analog meters are a little harder to read than digital, but I suspect their accuracy far exceeds digital, especially inexpensive ones. Also, analog meters don't need batteries.

The bias meter I have also pulled up a trace on the LED that connected the decimal select. During diagnosis I tugged on the wires gently and it pulled the trace right out.

[thomasdj]

Thanks:)

The meter works now...however this coursed a new problem..oscillation:S

I will make a new treat on this one..

Thomas